Known drive systems for hybrid vehicles including an engine and a motor generator which serve as a drive source are disclosed. For example, JP2009-208565A (hereinafter referred to as Patent reference 1) discloses a hybrid vehicle including an engine, a motor generator, a first clutch (front clutch) for transmitting a torque between the engine and the motor generator and for disconnecting the engine and the motor, and a multi-stage transmission (a multi-stage automatic transmission) provided between the motor generator and driving wheels. According to the drive system for hybrid vehicle of this kind, generally, a clutch and a transmission are automatically controlled by an electronic control unit (ECU). When brakes are applied, the hybrid vehicle releases the clutch to disconnect the engine from a drive force transmitting portion including the motor generator. Thus, a regenerative braking force is applied to the vehicle by a regenerative power generation of the motor generator to reduce speed, and a loss of an electric energy by an engine friction at the time is reduced to ensure high regenerative efficiency to improve fuel efficiency.
Further, when the regenerative power generation ends, the clutch is engaged so that the vehicle travels by an output torque from the engine and a control for matching rotation speeds of a driving side and a driven side is conducted. More particularly, with a general clutch, the clutch is controlled to be a half-engaged state in a state where a difference in rotation speeds exists between the driving side and the driven side, and the difference in rotation speeds gradually reduces by a frictional sliding movement so that the driving side and the driven side synchronously rotate. According to the hybrid vehicle, because the driving sources (the engine and the motor) are positioned at opposite sides of the clutch, respectively, a significant level of shock is likely to occur when the frictional sliding movement is performed in a state where the difference in rotation speeds exists. In order to avoid the foregoing drawbacks, generally, a control for matching rotation speeds in which a rotation speed of at least one of the driving sources is controlled prior to starting an engaging operation of the clutch so that the difference in rotation speeds is assumed to be equal to or less than a threshold value (in other words, the driving sources provided at both sides of the clutch are substantially synchronously rotated) is conducted.
According to the known drive system for hybrid vehicle, for example disclosed in Patent reference 1, when a driver steps on an accelerator pedal to accelerate the vehicle in a state where the clutch is released to generate the regenerative braking force by the regenerative power generation, in many times, an engaging operation of the clutch and a downshift operation of the transmission are successively requested. In those circumstances, according to the known drive system for hybrid vehicle, following the requested order, first, the engaging operation of the clutch along with matching the rotation speeds is controlled, then successively, the downshift operation is controlled in a state where the vehicle is driven by the engine and the motor. According to the known control method, a longer time is required from starting an operation of the accelerator pedal until the vehicle starts accelerating, which gives the driver a feeling that there is a time lag.
A need thus exists for a control device for a hybrid vehicle drive system, which is not susceptible to the drawback mentioned above.